Head-mounted projection display using reflective microdisplays

ABSTRACT

The present invention relates generally to a head-mounted projection display, and more particularly, but not exclusively to a polarized head-mounted projection display including a light engine and a compact, high-performance projection lens for use with reflective microdisplays.

RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 13/955,076 filed on, Jul. 31, 2013, which is divisional applicationof U.S. application Ser. No. 12/863,771 filed on Oct. 29, 2010, which isa 371 application of International Application No. PCT/US2009/31606filed Jan. 21, 2009, which claims the benefit of priority of U.S.Provisional Application No. 61/011,789, filed on Jan. 22, 2008, theentire contents of which applications are incorporated herein byreference.

GOVERNMENT RIGHTS

This invention was made with government support under governmentcontract IIS0534777 awarded by the National Science Foundation. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to a head-mounted projectiondisplay, and more particularly, but not exclusively to a polarizedhead-mounted projection display including a light engine and a compact,high-performance projection lens for use with reflective microdisplays.

BACKGROUND OF THE INVENTION

The head mounted projection display (HMPD), as an alternative to theconventional eyepiece-based head mounted display (HMD), has attractedmuch interest in recent years, because it offers the ability to design awide field of view (FOV), low distortion and ergonomically compactoptical see-through head mounted display (OST-HMD). Like most OST-HMDs,however, one of the limiting factors for the HMPD technology is its lowimage brightness and contrast, which limits the feasibility to applysuch information to outdoor or well-lit indoor environments such asoperating rooms. Due to the multiple beamsplitting through abeamsplitter and low retroreflectivity of typical retroreflectivematerials, the overall efficiency of a HMPD is around 4%. For instance,with a miniature backlit active matrix liquid crystal display (AMLCD) asthe image source, the luminance of the observed image is estimated to be4 cd/m², while the average luminance of a well-lit indoor environment isover 100 cd/m². As a result, the low-brightness image of HMPDs willappear washed out in such well-lit environments. In fact, most opticalsee-through HMDs, including HMPD, are typically operated under a dimmedlighting condition.

To address this problem, a polarized head-mounted projection display(p-HMPD) was proposed (H. Hua and C. Gao, “A polarized head-mountedprojective displays,” Proceedings of 2005 IEEE and ACM InternationalSymposium on Mixed and Augmented Reality, pp. 32-35, October 2005) and aprototype based on a pair of transmissive AMLCDs was designed recently(H. Hua, C. Gao “Design of a bright polarized head-mounted projectiondisplay” Applied Optics, Vol. 46, Issue 14, pp. 2600-2610, May 2007). Apair of 1.3″ color AMLCDs was used as the image sources which have aresolution of (640*3)*480 pixels. 1.4″ Alphalight™ RGB LED panels(Teledyne Inc., Los Angeles, Calif.) were used as the backlightingsources. By carefully manipulating the polarization states of the lightpropagating through the system, a p-HMPD can potentially be three timesbrighter than a traditional non-polarized HMPD design using the samemicrodisplay technologies. A schematic design of a monocular p-HMPDconfiguration is illustrated in FIG. 1.

The image on the LCD display is projected through the projection lens,forming a real intermediate image. The light from the LCD is manipulatedto be S-polarized so that its polarization direction is matched with thehigh-reflection axis of the polarized beamsplitter (PBS). After theprojected light is reflected by the PBS, it is retroreflected back tothe same PBS by a retroreflective screen. The depolarization effect bythe retroreflective screen is less than 10% within ±20 degrees and isless than 20% up to ±30 degrees. As a result, the retroreflected lightremains dominantly the same polarization as its incidence light. Inorder to achieve high transmission through the PBS after the light isretroreflected back, a quarter-wave retarder is placed between the PBSand the retroreflective screen. By passing through the quarter waveretarder twice, the incident S-polarized light is converted toP-polarization and transmits through the PBS with high efficiency. Thusthe projected image from the microdisplay can be then observed at theexit pupil of the system where the eye is placed.

However, since a transmissive LCD microdisplay has a low transmissionefficiency of around 5%, the overall performance of the first p-HMPDprototype is still unsatisfactory in a well-lit environment.Furthermore, owing to its inherent low pixel fill factor, a transmissiveAMLCD microdisplay typically has a relatively low resolution.Accordingly, it would be an advance in the field of head-mountedprojection displays to provide a head-mounted projection display whichhas higher luminance while maintaining high contrast.

SUMMARY OF THE INVENTION

In one of its aspects, the present invention provides a compact,telecentric projection lens for use in a head-mounted projection displaysystem. The projection lens may include a plurality of lens elementsconfigured to have an overall length that is no more than about twotimes the effective focal length of the projection lens. In addition,the plurality of lens elements may be configured so that the projectionlens is telecentric in image space. As one measure of the compactnessthe projection lens may have an overall length that is no more thanabout 85% larger than the effective focal length of the projection lens.Further, the projection lens may have a back focal length that is about40% larger than the effective focal length. To further facilitate theuse of the projection lens in a head-mounted display, the projectionlens may also be lightweight and have a ratio of the weight of theprojection lens to the square of the F-number of the projection lens ofless than about 2 to 1. For example, in one configuration the presentinvention provides a projection lens with a F-number of 2.1 and a weightof only 8.2 g.

In another of its aspects the present invention provides a telecentricoptical illumination system for use with a reflective microdisplay. Theillumination system may include a source of optical radiation having afirst polarization state and a polarized beamsplitter disposed at alocation to receive optical radiation from the optical radiation source.The beamsplitter may further be oriented relative to the source ofoptical radiation to reflect the received optical radiation. Inaddition, the illumination system may include an optical retarderdisposed at a location relative to the beamsplitter to receive opticalradiation reflected by the beamsplitter and oriented to convert thepolarization state of the received optical radiation to be circularlypolarized. A reflector having optical power may also be disposed at alocation to receive the circularly polarized optical radiation from theoptical retarder and to reflect the received optical radiation backthrough the retarder and beamsplitter. The reflector may be a concavespherical reflector. Still further, in order to provide an opticalillumination system that is telecentric in image space, the illuminationsystem may include a pupil disposed at the source of optical radiation.

In yet another of its aspects, the present invention provides ahead-mounted projection display system which may include theaforementioned illumination system, a microdisplay disposed onereflector focal length away from the reflector for receiving opticalradiation from the reflector, and a telecentric projection lens. Theprojection lens may assume the configuration of the aforementionedprojection lens.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary and the following detailed description of thepreferred embodiments of the present invention will be best understoodwhen read in conjunction with the appended drawings, in which:

FIG. 1 schematically illustrates a polarized HMPD system comprising atransmissive AMLCD and retroreflection screen;

FIG. 2 illustrates the luminance distribution of an exemplary LEDilluminator used in a light engine of the present invention;

FIG. 3 schematically illustrates an exemplary design of a light enginein accordance with the present invention;

FIG. 4 illustrates a prototype of a mirror based light pipe of the lightengine of the present invention;

FIG. 5 schematically illustrates an exemplary design of the taperedlight pipe of FIG. 4;

FIGS. 6A, 6B illustrate the illuminance distribution on the microdisplayprovided by the light engine of FIG. 3, without and with light pipe,respectively;

FIG. 7 illustrates the layout of an initial lens used as a startingpoint in the design of a projection lens of the present invention;

FIGS. 8A, 8B illustrate a lens layout and MTF performance, respectively,for an optimized version of the lens of FIG. 7;

FIGS. 9A, 9B illustrate a lens layout and MTF performance, respectively,for further optimized version of a projection lens of the presentinvention which is more compact than the lens of FIG. 8;

FIG. 10 illustrates the layout of the final design of an exemplaryprojection lens of the present invention;

FIGS. 11A, 11B illustrate the diffraction efficiency versus radius anddiffraction efficiency versus wavelength, respectively, for theprojection lens of FIG. 10;

FIGS. 12A, 12B illustrate a spot diagram and ray fan plot, respectively,for the projection lens of FIG. 10;

FIGS. 12C-12E illustrate the longitudinal spherical aberration,astigmatism, and distortion at 450 nm, 550 nm, 650 nm, respectively, forthe projection lens of FIG. 10;

FIG. 12F illustrates the MTF performance for the projection lens of FIG.10;

FIGS. 13A, 13B illustrate a front view and a side perspective view,respectively, of a prototype of a polarized head-mounted projectiondisplay of the present invention; and

FIG. 14 schematically illustrates polarized head-mounted display inaccordance with the present invention which includes the light engine ofFIG. 3 and projection lens of FIG. 10.

DETAILED DESCRIPTION

In order to realize the above-mentioned benefits afforded by areflective microdisplay 120 in the context of a head-mounted projectiondisplay, suitable optical designs are required for both a light engine100, to deliver uniform, telecentric illumination to the microdisplay120, and for a telecentric projection lens 300 to collect the lightreflected by the microdisplay 120 for delivery to the user, FIG. 3. Inaddition to the requirements of telecentricity and uniformity, the lightengine 100 and projection lens 300 must be sufficiently compact andlightweight to permit their use as part of the head-mounted system. Inthis regard, in one of its aspects, the present invention provides acompact, telecentric light engine 100 that includes a light pipe 110 andpolarization manipulating components 104, 108 to increase the overalluniformity and brightness, respectively, of the light delivered to areflective microdisplay 120, FIG. 3. In another of its aspects, thepresent invention provides a telecentric projection lens 300 comprisingall plastic components having aspheric surfaces and a diffractiveoptical element to yield a lens 300 that is simultaneously compact,telecentric, and lightweight, FIG. 10. Together, the light engine 100,reflective microdisplay 120, and projection lens 300 provide a polarizedHMPD in accordance with the present invention, FIG. 14.

Turning now to the overall system, design began by selection of anexemplary microdisplay 120 from which the performance requirements ofboth the light engine 100 and projection lens 300 may be derived. Majorproperties of several candidate microdisplay technologies wereevaluated, including AMLCD, organic light emitting displays (OLEDs),liquid crystal on silicon (LCOS), and ferroelectricLiquid-crystal-on-Silicon (FLCOS) were considered. For use in theexamples provided below, the SXGA-R2D FLCOS microdisplay kit (ForthDimensional Displays Limited, Birmingham, UK) was selected as anexemplary microdisplay for use in the present invention due to its highresolution, optical efficiency, and compactness. However, though a FLCOSmicrodisplay is used for illustrative purposes in the designs presentedbelow, it is understood that the present invention is not limited tosuch a microdisplay, but may be used in conjunction with suitable otherdisplays, such as, reflective microdisplay types like LCOS, MEMS, TexasInstruments DLP, and so forth, for example.

The usage of a FLCOS microdisplay 120 makes the prototype design of aHMPD quite different from previous designs of HMPD optics. One of thekey differences is the requirement for the custom-designed light engine100 to illuminate the microdisplay 120. The FLCOS microdisplay 120 isconsidered as the combination of a mirror and an electrically switchablequarter-wave retarder formed by the liquid crystal layers. The FLCOSmicrodisplay 120 works most efficiently when the illumination rays arenormally incident upon the display surface. To ensure the high contrastof the output image, it is recommended to limit the incident angle tothe range of ±16 degrees, which imposes a critical requirement on thedesign of both the light engine 100 and projection lens 300. The keyrequirements for the light engine 100 include: (1) that the illuminationsystem be image-space telecentric to ensure that for every pixel on thedisplay surface the incident chief ray is normal to the display surface;and (2) that the cone angle of the ray bundle is smaller than 16degrees.

Likewise, owing to the reflective nature of the microdisplay 120, theoutput ray bundles from every pixel of the microdisplay 120 aretelecentric with a cone angle smaller than 16 degrees. To efficientlycollect rays from the microdisplay 120 and form a projected image withuniform illumination, the projection lens 300 must be image-spacetelecentric. To the contrary, a projection system using backlit AMLCDs,which have a relatively large viewing angle, and thus relaxedrequirement for the angle of incidence ray bundles, can relax thetelecentric constraint to gain compactness.

TABLE 1 Specification of microdisplay and LED panel ParametersSpecifications FLCOS microdisplay Diagonal Size 22.3 mm Active area17.43 × 13.95 mm Resolution 1280 × 1024 pixels Pixel size 13.6 μm Colortechnique Field sequential color LED panel Body Dimensions 18.4 × 14.1mm Active area 8.4 × 6.5 mm Weight 4 ± .5 grams Luminance 34800 (cd/m²)Color Coordinates Red: x = .67-.43, y = .27-.33 Green: x = .14-.28, y =.64-.73 Blue: x = .11-.15, y = .04-.10 Power 340 mWDesign of a Compact Light Engine

Light source selection is the first step in the light engine design.Based on the design requirement of a HMPD, there are several constraintson the source selection and the light engine design. First, safety is aprimary concern in any head-mounted device. Therefore sources with lowpower consumption and low heat dissipation are highly desired. Secondly,compactness and low weight are always critical for HMD systems. Finally,in order to generate an image with high brightness and uniformity acrossthe whole FOV, the illumination on the microdisplay 120 should beuniform and bright. A 0.5″ Alphalight color LED panel 102 (TeledyneInc., Los Angeles, Calif.) is selected for the p-HMPD prototype design.The LED panel 102 is compatibly driven by the color sequential techniqueof the FLCOS displays. Table 1 summarizes the major specifications ofthe microdisplay 120 and LED panel 102. The luminance distribution ofthe LED panel 102, FIG. 2, is relatively uniform within 18 degrees ofthe emitting angle. With the requirement of image-space telecentricityon the light engine 100, a compact light engine design in accordancewith the present invention is shown in FIG. 3.

The basic idea of the design of the light engine 100 is to place the LEDpanel 102 at the front focal point of a positive lens (or reflector) andthe microdisplay 120 at the back focal point. In order to make a compactsystem, a concave spherical reflector 106, a PBS 104 and a quarter waveretarder 108 are used to fold the length of the system in half. Themicrodisplay 120 is disposed at the conjugate position of the LED panel102, and both the microdisplay 120 and LED panel 102 are at the focalpoint of the reflector 106. With a polarizer 114 in front of the LEDpanel 102, S-polarized light from the LED panel 102 is transmittedthrough the polarizer 114 and is reflected by the PBS 104. Aquarter-wave retarder 108 is placed between the reflector 106 and PBS104 and its fast axis is at 45 degrees with the polarization directionof S-polarized light. By passing through the retarder 108 twice, thereflected light by the reflector 106 becomes P-polarized light and willtransmit the PBS 104 to illuminate the microdisplay 120 with highefficiency.

In this design, the LED panel 102 itself can be taken as the pupil ofthe system to form an image-space telecentric system, where the raybundle received by the microdisplay 120 is symmetric with the displaynormal. With both LED panel 102 and microdisplay 120 at the focal pointof the reflector 106, the light distribution on the microdisplay 120 isthe Fourier transform of that of the LED panel 102. Thus the spatialdistribution on the microdisplay 120 can be derived as

$\begin{matrix}{{E_{display}\left( {x,y} \right)} = {{L*\Omega} = {{L_{LED}\left( {\arctan\left( \frac{\sqrt{x^{2} + y^{2}}}{f} \right)} \right)}*{\frac{S_{LED}}{f^{2}}.}}}} & (1)\end{matrix}$where E_(display)(x,y) is the illuminance at (x,y) on the displayassuming the center of the display is at the origin, L_(LED)(θ) is theluminance of LED panel 102 as a function of angle, S_(LED) is the areaof the LED panel 102 and f is the focal length of the reflector 106.Across the microdisplay 120, the ratio of the luminance at the center ofthe microdisplay 120 to that at the edge is

${L_{LED}\left( {0{^\circ}} \right)}/{L_{LED}\left( {\arctan\left( \frac{D}{2f} \right)} \right)}$while D is the diagonal size of the microdisplay 120. To get betteruniformity on the microdisplay 120, a reflector 106 with larger focallength is preferred. But meanwhile, larger focal length will result in aless compact structure and a smaller solid angle with lower luminanceefficiency. Considering all these factors, a reflector 106 with 35 mmfocal length and 35 mm diameter was selected. As a result, the ratio ofthe maximum luminance to the minimum luminance on the microdisplay 120is 1:0.82, and the cone angle of the ray bundle on the microdisplay 120is within 8.6 degrees. The light within the cone angle of 18 degreesemitted by the LED panel 102 can be collected by the reflector 106 toilluminate the microdisplay 120 while the light emitted at a largerangle is wasted.

In order to further improve the light efficiency and uniformity of thelight engine 100, a mirror-based tapered light pipe 110 was designed torecycle the light with emission angles larger than 18 degrees. FIG. 4shows a prototype design of the light pipe 110. It is composed of fourmirrors 112, each of which is tilted by an angle with respect to the LEDsurface, forming a truncated pyramid shape. The light emitted from theLED panel 102 with large angles will be reflected by the enclosingmirrors 112. After reflection, more rays from LED panel 102 can becollected by the reflector 106 to illuminate the microdisplay 120. Toget the best performance of the light engine 100, both tilt angle, α,and length, t, of the light pipe mirror 112, as shown in FIG. 5, shouldbe optimized.

Numerical Simulation

To determine the parameters of the tapered light pipe 110 and to examinethe light efficiency and uniformity of the light engine 100, the lightengine 100 was modeled using LightTools® (Optical Research Associates,Pasadena, Calif.). In the simulation, the total power of the source wasset to be 1 lumen. A light receiver was placed on the microdisplay 120to estimate the efficiency of the light engine 100 and to evaluate thelight distribution on the microdisplay 120. Through the simulation, itis shown that the light engine 100 has higher uniformity and lightefficiency when the tilt angle, α, of the mirror is 18 degrees. Bybalancing the performance and space constraint of the light engine 100,the mirror length, t, is selected to be 8 mm. FIG. 6B shows the outputilluminance distribution on the microdisplay 120 for a system with themirror based light pipe 110, and FIG. 6A shows the output illuminancedistribution on the microdisplay 120 for the same system as FIG. 6B butwithout the light pipe mirrors 112 in place. Hence, comparison betweenFIGS. 6A, 6B illustrates the improvement in performance of the lightengine 100 due to the presence of the light pipe mirrors 112.

As indicated by the simulation results, with a mirror based light pipe110, the light efficiency has increased from 8.93% to 12.3%, and thenon-uniformity, quantified by the average standard deviation of theilluminance distribution across the display area, has reduced from 5.61%to 2.15%. Thus, the system with the light pipe 110 has higher efficiencyand better uniformity than without.

Design of a Compact, High-Performance Projection Lens

Based on the design of the light engine 100, a lightweight and compactimage-telecentric projection lens 300 is designed. In this section, thedesign process of the projection lens 300 is described and theperformance of the projection lens 300 is analyzed.

TABLE 2 Design targets for the projection lens Parameter SpecificationEffective focal length 21.6 mm Entrance pupil 10 mm Image mode Imagespace telecentric OAL <40 mm OAL/BFL <1.85 BFL 30.5 mm FOV 55°Wavelength range 486-656 nm Distortion <4% over FOV Weight <15-20 gramsProjection Lens Specification

Although projection optics do not scale as much with the increase of FOVas eyepiece-type optics, which makes designing wide FOV, opticalsee-through HMPD systems relatively easier than conventional HMDs, thereare a few factors in HMPD systems that impose limits on the FOV. Firstof all, the use of a planar PBS 116 or a regular beamsplitter in frontof the eye, which is oriented at 45 degrees with the optical axis, setsup the FOV upper limit of 90 degrees. Furthermore, a wide FOV requires alarge size of PBS 116 and retarder 118 and consequently challenges thecompactness and lightweight of the display system. The limit ofallowable PBS and retarder dimensions is set by the interpupilarydistance (IPD), which is in the range of 55 to 75 mm for over 95% of thepopulation. Thirdly, previous investigation on retroreflective materialsshows that the retroreflectance of currently available materials dropsoff significantly for light incident at angles beyond ±35°. A FOV beyond70 degrees will inevitably cause vignetting-like effect and compromiseimage uniformity. Finally, the angular resolution of the microdisplay120 degrades with the increase of the FOV. Taking into account thesefactors, a target was set to design the projection system with a FOV of55 degrees, which corresponds to an effective focal length (EFL) of 21.6mm for the selected FLCOS microdisplay 120.

In addition to being image-space telecentric, the projection lens 300must have a large back focal length (BFL) to ensure enough space for thePBS 104 which is placed between the microdisplay 120 and projection lens300. Based on the light engine design, the BFL is chosen to be at least30.5 mm. Thus this projection lens 300 is also a reverse telephoto lens.(It is understood that, in the designs below, the microdisplay 120 islocated at the image plane, hence the reason that the BFL represents thespace allotted for the PBS 104 placed therebetween.)

Since a user's eye is positioned at the conjugate position to theentrance pupil of the projection lens 300, the entrance pupil diameteris very critical for comfortable observation. Typically it is suggestedthe pupil diameter of the projection system for HMPDs should be around10-12 mm. This range of pupil size allows an eye swivel of about ±21° upto 26.5° within the eye sockets without causing vignetting or loss ofimage with a typical 3-mm eye pupil in the lighting conditions providedby HMPDs. Furthermore, it allows a ±5 mm to 6 mm IPD tolerance fordifferent users without the need to mechanically adjust the IPD of thebinocular optics. Considering the short focal length of the optics, atarget entrance pupil with a diameter of at least 10 mm was selected,which leads to a projection system with an F/# of 2.16.

A lens 10 from Nanba U.S. Pat. No. 6,236,521 was used as the startingpoint of the design, with FIG. 7 showing the layout. The starting lens10 was designed for a digital projector, and is reverse-telephoto andtelecentric. Unlike a double Gauss lens, the starting lens 10 has anasymmetric structure relative to the stop because of the telecentricrequirement in image space. This five-element starting lens 10 offers afull FOV of 65 degrees with an F/# of 2.5. Among the five glasselements, a doublet 12 is used, and the front surface 14 of the lastelement 16 is aspheric to help correct spherical aberration. The ratioof the BFL to the EFL of the lens 10 is 1.13 and the ratio of theoverall length (OAL. As used herein, OAL refers to the distance from thefirst surface of the first optical element to the last surface of thelast optical element) of the optics to the EFL is 3.15. The ratio of theexit pupil distance to the EFL is 13.6, which makes the lens 10telecentric in image space. By scaling and several cycles ofoptimization with CODE V® (Optical Research Associates, Pasadena,Calif.), a new starting lens 210 was obtained to meet the first-orderdesign targets of 21.1 mm EFL, 30 mm BFL and 68 mm OAL, as shown in FIG.8A and Table 3. The full FOV is set to be 55 degrees. As shown in FIG.8B, the MTF of the lens 210 is around 30% at the spatial frequency of37-lp/mm, which shows acceptable performance as a first-order startingpoint for the design.

TABLE 3 Numerical values for lens of FIG. 8. Surface Surface Lens No.type Radius Thickness material 1 Sphere 151.9831514 7.199 772499.496 2Sphere 18.31108732 18.74851 3 Sphere 37.26417226 4.646097 772499.496 4Sphere 51.05918244 9.065786 stop Sphere 1.00E+18 11.41511 6 Sphere14.31228599 4.642326 846660.238 7 Sphere 121.7152043 6.393417 677900.5538 Sphere 20.60785676 0.1 9 Asphere 60.8179454 5.789755 772499.496 10 Sphere 38.94828517 29.99994 Asphere parameters for surface 9 4th OrderCoefficient (A) −8.95E−06 6th Order Coefficient (B) 6.83E−09 8th OrderCoefficient (C) −1.56E−11 10th Order Coefficient (D) 2.28E−14

The projection lens 210 is rotationally symmetric, and thus theoptimization is only necessary over half of the full FOV for the radialdirection. Three representative wavelengths (i.e., 486 nm, 589 nm and656 nm) were set with the weights of 1, 2 and 1, respectively. Fivefields, corresponding to 0, 7°, 14°, 21° and 27.5°, respectively, wereused in the optimization process to represent the half FOV. The weightsof the five fields were adjusted in the optimization process to balancethe MTF performances across the entire FOV. During the optimization, allsurface curvatures, surface thicknesses, and coefficients of asphericsurfaces were set to be variables. Several constraints were set tosatisfy the specifications of the overall system and each individuallens element, including the EFL, BFL, OAL, distortion requirements, andthe center thickness of individual elements, for example. Thetelecentric requirement was satisfied by setting the exit pupil distanceto be at least 210 mm from the image plane. This distance corresponds toa deviation of the chief ray by 3° from a perfectly telecentric system,which yields a good balance between the overall optical performance andthe system compactness considering the difficulty in designing aperfectly telecentric lens with a short OAL.

One of the major problems of the lens 210 in FIG. 8A is its compactness:the OAL is too large for a head-mounted system and a more compactsolution is needed. This initial lens 210 was gradually optimized byadjusting the parameter constraints and field weights through a localoptimization approach. While the OAL was reduced down to about 40 mm inthe process of optimization, the overall performance was degraded aswell. In order to further improve its performance, the back surface 211of the first lens element 212 was set to be aspheric, which helped tocorrect most of the spherical aberration. After gradual optimization, asystem was obtained with satisfactory performance. The resulting lenslayout and MTF are shown in FIGS. 9A, 9B.

This lens 220 is composed of five glass elements 221-225 (with asphereson two surfaces 227, 229) and weighs about 38.7 grams, which needs to besignificantly reduced to obtain a lightweight p-HMPD system. Plasticmaterials were selected to replace the glass elements 221-225.Considering that the density of glass is about three times of mostplastics, it is expected that the weight of the lens 220 would drop toaround 10 grams. The drawback of using the plastic materials is alsoobvious. While there are many choices of optical glass, from low to highrefractive index and low to high Abbe number, for plastics only verylimited number of materials are available for diamond turningfabrication.

The initial target was to replace the glass elements 223-225 on theright side of the stop with the plastics, since they contributed most ofthe weight due to their large aperture. Polystyrene with low Abbe numberand Cyclic Olefin Polymer (COC) with relatively high Abbe number wereselected to replace the glass materials of the doublet 223, 224. Afterthis replacement, the optimization was rerun to achieve the desiredspecifications of the lens 220. Since the last element 225 has thehighest optical power among all the elements in the lens 220 and highAbbe number, COC was selected for the last element 225. Unfortunately,after the plastic replacement the system had much worse performancecompared with the system before replacing the last glass element 225.Chromatic aberration dominates the resulting system. To effectivelycorrect the residual chromatic aberration, a diffractive optical element(DOE) was added to the system.

A DOE can be viewed as a material with large dispersion but opposite insign to conventional materials, i.e., the Abbe number of a DOE isapproximately −3.5 for the visible spectrum. This DOE with negative Abbenumber would help to correct the chromatic aberration. The substrateshape of a diffractive surface DOE can be spherical, planar being aspecial case, or aspheric. The commonly used orders of diffraction are0, −1, or 1. The +1 order of diffraction was adopted. The diffractivesurface can be used on any of the lens surfaces as long as it can helpto correct the chromatic aberration. However, after a few trials, it wasfound to be most effective to place the diffractive surface on the leftsurface 227 of the last element 225. The DOE quadratic coefficient wasset to be a variable up to 12 orders in the optimization process.Finally, to further reduce the lens weight, the first two glass elements221, 222 on the left of the stop were replaced with Acrylic andPolystyrene, respectively.

Considering the fact that a doublet requires higher cost in thefabrication process, the doublet 223, 224 was split into two singleelements. The split of the doublet 223, 224 also offered extra freedomin the optimization, and helped improve the overall performance.Finally, through several rounds of optimization, a telecentric lens 300with an OAL of 34 mm and a total weight of 8.2 grams was obtained. Theratio of the weight of the lens to the square of the F-number was 1.86to 1. FIG. 10 shows the layout of the final design of the projectionlens 300, and Tables 4-9 shows the design parameters and first orderproperties of the lens 300.

TABLE 4 Numerical values for the lens of FIG. 10. Surface Surface No.type Radius Thickness Material 1 Sphere 54.31648697 2 Acrylic 2 Asphere11.67695001 1.31979 3 Sphere 29.18891338 2.71793 Polystyrene 4 Sphere−53.75253258 3.106908 5(stop) Sphere Infinity 6.769705 6 Asphere−9.34878697 2.5 Polystyrene 7 Sphere −109.22569406 0.2 8 Sphere−93.99209709 7.088732 Cyclic Olefin Polymer 9 Sphere −14.34048220 0.110  Asphere 36.51968881 8.196935 Cyclic Olefin Polymer 11  Sphere−22.22836849 30.5 image Sphere Infinity 0

TABLE 5 Numerical values for aspheric surfaces of the lens of FIG. 10.Element Focal length (mm) 1^(st) element −30.7666976612 2^(nd) element32.4497012224 First two elements −7837.900440744 3^(rd) element−17.48717528244 4^(th) element 30.96961073305 5^(th) element25.97749355667

TABLE 6 Numerical values for aspheric surfaces of the lens of FIG. 10.Surface Surface 2 Surface 6 Surface 10 Conic Constant (K) 1.19517273E+00 2.87739760E−01 −1.73713081E+01 4th Order Coefficient (A)−9.17398530E−05 1.55078027E−04  1.87808847E−07 6th Order Coefficient (B)−6.00685014E−06 1.60075455E−06 −1.36998214E−07 8th Order Coefficient (C) 2.67960130E−07 −1.10443177E−07   7.55859302E−10 10th Order Coefficient(D) −9.29841239E−09 4.67064799E−09 −2.04509691E−12 12th OrderCoefficient (E)  1.54727549E−10 −8.56490951E−11   2.26702680E−15 14thOrder Coefficient (F) −1.16675172E−12 6.81530567E−13 −1.54585020E−19

TABLE 7 Numerical values for the DOE of the lens of FIG. 10. DiffractionOrder = 2 Construction Wavelength = 550 R**2 (C1) −1.06653506E−03 R**4(C2)  4.09441040E−06 R**6 (C3) −4.17021652E−08 R**8 (C4)  2.36559269E−10R**10 (C5) −6.59579639E−13 R**12 (C6)  7.29317846E−16

TABLE 8 Numerical values for optical materials of the lens of FIG. 10.Acrylic Polystyrene Cyclic Olefin Polymer Trade Name Plexiglas StyronZeonex nf (486.1 nm) 1.497 1.604 1.537 nd (589 nm) 1.491 1.590 1.530 nc(656.3 nm) 1.489 1.585 1.527

TABLE 9 First order properties of the lens of FIG. 10 at infiniteconjugates. EFL 21.6000 BFL 30.5000 FFL 4.5965 F No 2.1600 Imagedistance 30.5000 OAL 34.0000 Paraxial image height 11.2442 Paraxialimage Ang 27.5000 Ent pupil diameter 10.0000 Ent pupil thickness 7.0384Exit pupil diameter 88.4570 Exit pupil thickness −160.5672

The lens 300 includes five lens elements 310, 320, 330, 340, 350 orderedleft-to-right from object space to image space. The first two lenselements 310, 320 form a first lens group 315 having very little opticalpower (e.g., the focal length of the lens group 315 is two orders ofmagnitude larger than that of the lens 300, see Tables 5, 9) disposed tothe left of the stop. The first lens element 310 is a negative meniscuslens having two surfaces that are concave towards image space, with thesurface 311 closest to image space being aspheric. The second lenselement 320 is a positive lens disposed adjacent the first lens element310. The third through fifth lens elements 330, 340, 350 are disposed onthe right side of the stop, with the third element 330 closest to thestop being a negative lens and the fourth and fifth elements 340, 350closest to the image plane being positive lenses. The first surface 331of the third lens element 330 closest to the stop is aspheric as is thefirst surface 351 of the fifth lens element 350. In addition, the firstsurface 351 of the fifth lens element 350 includes the aforementionedDOE.

Performance Analysis of the Projection Lens

The diffraction efficiency of a DOE drops as its physical featuresbecome finer near the edge. FIG. 11A shows the diffraction efficiency asa function of the radius of diffractive surface at the designedwavelength 550 nm. The overall efficiency varies from 98.7% at thecenter to 98.5% at the edge. The diffraction efficiency is alsowavelength dependent. FIG. 11B plots the diffraction efficiency as afunction of wavelengths as well as the levels of the binary masks (i.e.,2, 4, 8, 16). Level 16 could be an accurate prediction for the KinoformDOE using diamond turning method for fabrication. It shows that thediffraction efficiency varies from 80% to 100% across the visiblespectrum.

The optical performance of the optimized lens 300 is assessed on theimage plane at the five representative field angles for three differentwavelengths. The spot diagrams are shown in FIG. 12A. The average RMSspot diameter across the FOV is around 16 μm, which is slightly largerthan the 13.6 μm pixel size to avoid pixellated artifacts. FIGS. 12C-Eshows longitudinal spherical aberration, astigmatism, and the distortioncurves. The longitudinal spherical aberration and astigmatism are wellbalanced, and the distortion of the system is limited within 4% acrossthe FOV. The MTF of the lens 300 is presented in FIG. 12F. The FLCOSmicrodisplay 120 has a threshold spatial frequency of 36.8-lp/mm given a13.6 μm pixel size (i.e. threshold spatial frequency=1/(2*pixel size)).The modulation is about 40% at 36.8-lp/mm across the whole FOV, whichmeans the performance of the system is currently limited by the displayresolution.

p-HMPD Prototype

With the design of the light engine 100 and projection lens 300complete, a prototype of the new p-HMPD was built, FIGS. 13-14. Thep-HMPD included the light engine 100, microdisplay 120, and projectionlens 300, as well as an additional PBS 116, quarter-wave retarder 118,and retro-reflective screen, which are disposed in the same relativepositions to provide the same function as the PBS, quarter-waveretarder, and retro-reflective screen of FIG. 1. Compared with thep-HMPD prototype using transmissive LCD microdisplays of FIG. 1, themounting of the optics in the new p-HMPD of FIG. 3 is more challengingfor the following reasons. First, the use of the light engine 100requires some extra space and weight. Second, the projection lens 300designed for the FLCOS microdisplay 120 is longer due to the image-spacetelecentric requirement and higher image quality requirement.

Considering both the ergonomic and aesthetic factors, the optics 100,120, 300, were mounted vertically so that the width of the helmet wasaround the average width of the adult head. In the vertical direction,the optics were mounted according to the shape of the head, and theassociated electronics were mounted on the top of the helmet. A maindrawback of the vertical mount is that a ghost image of the groundformed by the PBS 104 is overlaid with the projected image, which leadsto reduced image contrast. This problem, however, can be solved byblocking the optical path from the ground.

To make the system more compact and lighter, the mount of the lightengine 100 with the microdisplays 120 was fabricated separately and thenintegrated with the shell as a whole. The lens 300 position relative tothe microdisplay 120 is adjustable to provide a projected image withadjustable magnification.

The helmet shells were fabricated using rapid prototyping techniques, inwhich physical models are fabricated layer by layer directly from a 3DCAD model. The helmet shells were assembled and attached to anoff-the-shelf headband that offers head-size adjustment. The front andside views of the prototype are shown in FIGS. 13A, 13B, respectively.

These and other advantages of the present invention will be apparent tothose skilled in the art from the foregoing specification. Accordingly,it will be recognized by those skilled in the art that changes ormodifications may be made to the above-described embodiments withoutdeparting from the broad inventive concepts of the invention. It shouldtherefore be understood that this invention is not limited to theparticular embodiments described herein, but is intended to include allchanges and modifications that are within the scope and spirit of theinvention as set forth in the claims.

What is claimed is:
 1. A head-mounted projection display system,comprising: a microdisplay comprising a plurality of pixels; anillumination system and a projection system having a chief ray and a raybundle, each system in optical communication with one another and withthe microdisplay and each system telecentric in image space relative tothe microdisplay such that the chief ray incident at the surface of themicrodisplay is normal to the surface and the cone angle of the raybundle is less than 16°.
 2. The head-mounted projection display systemaccording to claim 1, wherein the microdisplay is reflective, and thebundle of rays is reflected from the microdisplay to be telecentric withthe cone angle smaller than 16°.
 3. The head-mounted projection displaysystem according to claim 2, wherein the incident chief ray is normal tothe microdisplay surface at each pixel of the microdisplay.
 4. Thehead-mounted projection display system according to claim 1, wherein theincident chief ray is normal to the microdisplay surface at each pixelof the microdisplay.
 5. The head-mounted projection display systemaccording to claim 1, wherein the cone angle of the ray bundle is lessthan 9°.
 6. The head-mounted projection display system according toclaim 1, wherein the illumination system comprises a light engine. 7.The head-mounted projection display system according to claim 6, whereinthe light engine comprises a mirror-based tapered light pipe.
 8. Thehead-mounted projection display system according to claim 1, wherein theillumination system includes a light source and the projection systemincludes a front focal point and a back focal point, and wherein thelight source is disposed at the front focal point and the microdisplayis disposed at the back focal point.
 9. The head-mounted projectiondisplay system according to claim 8, wherein the microdisplay isdisposed at a conjugate position to the light source.
 10. Thehead-mounted projection display system according to claim 1, comprisinga reflector having optical power, a beam splitter, and quarter waveretarder each disposed in between the illumination system and themicrodisplay.
 11. The head-mounted projection display system accordingto claim 10, wherein the microdisplay and a light source of theillumination system are disposed at a focal point of the reflector.